Yamaha G50 112

[blindboybenton]

Hi

Ive got a customer who brought the above mentioned amp from ebay. Its in great condition but a little noisy. Customer is using a Hagstrom with P90's with the treble turned up to about 6-7 . too hissy to record with. he also has an Epiphone 335 DOT much better though that but still noisy

Ive checked it out all voltages are correct Done the usual things ( Pots and jacks cleaning ) . Noisy is coming from preamp. I think its ok noise wise

Question does anyone know these amps or have one...... Are they noisy ?

Anybody has any idea's how i can reduce the noise ?
yamaha-g50-112-amplifier-schematics.pdf
BBB

[Enzo]

Don't plug ANY instrument into it. Turn it up. Now we are hearing only the noise of the amp itself. ANy different?

I expect some noise, don't know if yours has excess.

recording? How about a noise gate on the mixer feed.

I would expect hiss to come from the preamp rather than powr amp. But make sure. The preamp and power amp are on separate boards, looks like the orange wire caries the preamp signal to the PA. Try grounding the orange wire at the power amp. Kills the hiss? Don;t worry if it makes some hum, all we care is whether it kills the hiss.

On the preamp is a brightness control, does it affect the tone of the hiss? Also the volume control, does it turn down the hiss? i.e. there is no hiss with the volume at zero? There are three tone controls, do they affect the tone of the hiss? I would expect the OD to increase the hiss when it is turned on and up, is that true? And of course turning down the reverb, any difference?

Doing these things helps narrow down which stage is noisy. Even if the hiss is normal, it might be possible to reduce it with low noise transistors.

[blindboybenton]

Hi Enzo

Got it back on the bench again
With no input signal.. and volume up full is very quite. If i have a guitar plugged in and volume up full tones about 1/2 way.. its noisy. If i back off the treble it goes. The other tones have no real effect on the hiss.. If i disconnect the orange wire ( signal to power amp ) its quite.. If i ground the wire it kills the hiss
Brightness has no change on the hiss. If you turn the volume down with a guitar in the hiss goes away

many thanks for the help

BBB

[Enzo]

So that pretty much localizes it to the first stage, which is not surprising.

With an empty input jack, the first stage input is essentially grounded, and with a guitar but the guitar volume to zero, I bet it is also quiet?

Not a lot of options. There are voltages on the schematic, so are the voltages at source and drain of TR1 reasonably close?

TR1 and TR5 are the same type, you could trade places with the two and see if the other transistor is less noisy.


And it may just be the nature of the thing.

[blindboybenton]

Replaced the 2 x fets with new 2SK30a and some of the 2SC1000 for 2SC1815 and its done the job. Reduced the hiss by about a 1/3
The amp is now quieter than a marshall and Peavey KB100 in the workshop

Blindboybenton